Bacillary Layer Detachment (BALAD) is an optical coherence tomography (OCT) finding observed in many types of uveitis and retinal diseases. It refers to an OCT finding in which separation occurs at the level of the inner segment myoid of photoreceptors, forming a fluid-filled cavity within the retina.
Bacillary layer detachment (BALAD) is the separation of the bacillary layer from other retinal layers due to splitting within the inner segment (IS) myoid of photoreceptors, just after the external limiting membrane (ELM).
The term “bacillary” refers to the inner and outer segments (IS-OS) of photoreceptors. It was described by van Leeuwenhoek in the 1700s as “stratum bacillorum et conorum” (layer of rods and cones), and in the 1940s Polyak defined the bacillary layer as the IS-OS of photoreceptors.
Bacillary layer detachment was first reported by Mehta et al. in 2018. Previously, it had been individually reported as subretinal septa in Vogt-Koyanagi-Harada disease, subretinal fluid in ocular toxoplasmosis, and atypical outer retinal fluid in neovascular age-related macular degeneration, but no specific name was given until 2018.
Rod-cone layer detachment is not an independent disease but an OCT finding (sign) observed in multiple retinal and uveal diseases. It is important to identify and treat the underlying disease, and resolution of the detachment serves as an indicator of visual recovery.
Subjective symptoms associated with rod-cone layer detachment vary depending on the underlying disease, but the following are mainly reported.
Photoreceptor layer detachment presents findings similar to subretinal fluid (SRF). A well-defined round or oval elevation is observed at the fovea, with a yellowish-gray color, surrounded by a hypopigmented yellow ring. Common locations are as follows:
Fovea > Parafoveal > Peripapillary
The diagnosis of foveal detachment is confirmed by OCT. The following characteristic OCT findings are observed1).
| Examination | Findings |
|---|---|
| Fundus autofluorescence (FAF) | Center: hypoautofluorescence. Perilesional: hyperautofluorescent ring |
| Fluorescein angiography (FFA) | Hyperfluorescence due to dye pooling in the outer nuclear layer detachment (with marginal hypofluorescence in the late phase) |
| Indocyanine green angiography (ICGA) | Early hypofluorescence due to choroidal perfusion deficit may be observed |
| OCTA | Confirmation of subretinal neovascularization in neovascular age-related macular degeneration; decreased capillary blood flow in inflammatory diseases |
The yellow border on fundus photography, the hyperreflective ring on near-infrared imaging, the hypofluorescent boundary on late-phase FFA, and the angle of photoreceptor layer detachment on OCT all correspond to each other1).
Diseases that cause rod-cone layer detachment are diverse.
Inflammatory Diseases
Infectious: Toxoplasma chorioretinitis, tuberculous serpiginous choroiditis, endophthalmitis
Non-infectious: Vogt-Koyanagi-Harada disease, sympathetic ophthalmia, acute idiopathic maculopathy (AIM), posterior scleritis, acute posterior multifocal placoid pigment epitheliopathy (APMPPE)
Degeneration, Vascular, and Others
Degenerative diseases: Neovascular age-related macular degeneration, central serous chorioretinopathy
Vascular diseases: Hypertensive retinopathy, diabetic retinopathy, retinal vein occlusion, type 2 macular telangiectasia
Drug-induced: Dabrafenib (anti-BRAF drug), trametinib (anti-MEK drug)
OCT is essential for diagnosing photoreceptor layer detachment, and the following differential diagnosis is important.
| Feature | Photoreceptor layer detachment | Subretinal fluid (SRF) |
|---|---|---|
| Separation site | Within the inner segment myoid (MZ) of photoreceptors | Between RPE and neurosensory retina |
| Reflectivity of intra-cavitary material | High reflectivity (suggesting IS-OS debris + fibrin) | Lower reflectivity |
| Attachment to RPE | Separated IS-OS attached to RPE | RPE separated from neurosensory retina |
Bleeding within the photoreceptor layer detachment cavity is called “hemorrhagic photoreceptor layer detachment.” It is seen in cases of macular neovascularization. Dense hyperreflective material is observed within the cavity. It must be differentiated from subretinal hemorrhage, as the treatment differs1).
In retinoschisis, connecting fibers or strands are observed on OCT, which is useful for differentiation.
Ramtohul et al. (2021) reported a hemorrhagic bacillary layer detachment in a 55-year-old woman with proliferative MacTel 21).
OCT angiography confirmed subretinal neovascularization at the outer retinal level. OCT showed bacillary layer detachment, and the hyperreflective material within the cavity corresponded to the location of subretinal hemorrhage on fundus photography, suggesting heme accumulation. Follow-up OCT one month after a single intravitreal injection of ranibizumab (0.5mg/0.05mL) showed marked regression of the bacillary layer detachment. However, localized thinning of the ellipsoid zone and interdigitation zone persisted1).
This case suggests that strong fluid leakage from subretinal neovascularization and modification of the adhesion between photoreceptor outer segments and the RPE due to subretinal hemorrhage may have triggered the development of bacillary layer detachment1).
OCT examination is central to diagnosis. A cystoid separation cavity between the external limiting membrane (ELM) and the ellipsoid zone (EZ), with the posterior boundary remaining attached to the RPE, is a characteristic finding of bacillary layer detachment. To confirm the underlying disease, multiple tests such as fundus examination, FA, ICGA, and OCTA are combined for evaluation.
To resolve bacillary layer detachment, it is essential to identify and treat the underlying disease. There is no direct treatment for bacillary layer detachment itself.
The structure of the outer retina from the outside inward is as follows: retinal pigment epithelium (RPE), interdigitation zone (IZ), photoreceptor outer segments (OS), and photoreceptor inner segments (IS). The inner segment is further divided into the distal ellipsoid zone (EZ) and the proximal myoid zone (MZ), with the external limiting membrane (ELM) located on the inner side.
The MZ contains the Golgi apparatus, ribosomes, and endoplasmic reticulum, while the EZ contains densely packed mitochondria.
The structure of the inner segment myoid of photoreceptors is not as strong as the ELM or EZ, making it a zone of weakness prone to separation as seen in bacillary layer detachment.
According to Mehta et al., the following two factors are necessary for the formation of bacillary layer detachment:
The ellipsoid zone of photoreceptors adheres to the RPE/Bruch’s membrane complex, and accumulation of intraretinal fluid pushes up the ELM, creating a cleavage plane at the level of the inner segment myoid.
Sudden and rapid influx of intraretinal and subretinal fluid (retinal acute fluid accumulation: RAFA) is considered to be more involved in the development of bacillary layer detachment than inflammation of the underlying disease.
Choroidal ischemia is another possible pathology of bacillary layer detachment. Choroidal thickness is often increased in patients with bacillary layer detachment, which is associated with diseases that can complicate bacillary layer detachment, such as Vogt-Koyanagi-Harada disease, sympathetic ophthalmia, and APMPPE. The fact that bacillary layer detachment responds well to steroid therapy and plasma exchange also supports this theory.
Shear forces from subretinal hyperreflective material (SHRM) in neovascular age-related macular degeneration and shear forces from ocular trauma may also cause MZ separation1).
Photoreceptor layer detachment has been suggested as a potential biomarker for poor visual prognosis in Vogt-Koyanagi-Harada disease. Prospective studies are expected to verify its usefulness as a biomarker.
The widespread use of high-resolution OCT and swept-source OCT enables more detailed analysis of photoreceptor layer detachment. Integration with en-face OCT and OCTA is expected to improve the accuracy of evaluating the pathology of underlying diseases and assessing treatment efficacy.
Research is ongoing on the frequency and clinical significance of bacillary layer detachment in various diseases such as toxoplasmic chorioretinitis, APMPPE, and type 2 macular telangiectasia. The impact of the presence of bacillary layer detachment on treatment decisions also requires elucidation.
Resolution of bacillary layer detachment is associated with visual acuity recovery, but complete visual recovery is not always guaranteed. In the case reported by Ramtohul et al. (2021), localized thinning of the ellipsoid zone and interdigitation zone persisted after regression of bacillary layer detachment 1). Hemorrhagic bacillary layer detachment and the presence of subretinal hyperreflective material are risk factors for poor visual prognosis. Early treatment is most important for structural and functional recovery of photoreceptors.
